The present invention relates to a data transmission system wherein a plurality of transmission devices are interconnected through a transmission medium and exchange data therebetween.
A local area network (LAN) has been recently popular to allow data communications between a large number of transmission controllers (to be referred to as nodes hereinafter) through a transmission line as a transmission medium. Among various LAN communication schemes, a token passing system, for causing only the node receiving a token as a transmission right assignment instruction to have the transmission right, has advantages in that the system can control the transmission line occupied times of all nodes even when there is high traffic in the network without degrading transmission efficiency.
Token passing system networks are classified into a token ring system and a token bus system according to their system configrations. The LAN standards of these systems are proposed by IEEE Project 802.
In the token ring system the direction of token passing is determined. The next token destination is always the adjacent node which is physically determined.
A token ring system LAN is exemplified in FIG. 1. Transmission lines 100 to 160 are respectively nodes A to G constituting a LAN. Host devices A to G (200 to 260) such as work stations, facsimile systems or file servers are respectively connected to the nodes A to G (100 to 160). A token 1 can be accessed as follows.
(1) A node with transmission data monitors passing of the token while performing regenerative repeating of a reception signal string. If the priority of transmission data is higher than or equal to that of the token, access control token bits are updated to represent the updated frame leading. A bit string from the frame control to frame status is sent out.
(2) A node represented by a destination address in a frame determines whether the destination address coincides with the self node address. If a coincidence is established, the node copies the frame. Coincidence detection and frame copying are set in the frame status, and a response signal is sent back to a source node.
(3) The source node erases the received frame and detects a coincidence between the source address and the self node address to determine the return of the leading frame already sent through the destination nodes. Thereafter, the source node sends out a token to assign the transmission right to the next node.
A conventional token bus system LAN is shown in FIG. 2. The same reference numerals as in FIG. 1 denote the same parts in FIG. 2.
Since the conventional bus LAN commonly uses a single transmission line, it constitutes a logical ring so that the token is sent to each adjacent node one by one. Assume that the same logical ring as in FIG. 1 is formed. A token passing diagram formed by the logical ring is illustrated in FIG. 3. Each node detects a preceding node which sends a token thereto and a downstream node which receives the token therefrom according to MAC control commands "who-follow" and "set-successor". Each node also detects if a new node added to the network is present by sending a MAC control command "solicit-successor" and monitoring a response. If the new node is present, each node sets it in the logical ring.
If a plurality of additional nodes are present, the response signals are subjected to a conflict or contention. In order to prevent this, a MAC control command "resolve-contention" is used.
A maximum token occupied time of a node obtaining the transmission right is predetermined in each LAN system. The node holds the transmission right within the maximum token occupied time and sends the token to the downstream node.
Although the token passing systems are good transmission control systems, as described above, they poses the following problem. Because equipment is actually connected to each node in a LAN, equal token passing and equal token assignment within the maximum token occupied time, both of which are the characteristics of the token passing system, inevitably result in inconvenience.
In a system having both personal and host computers as equipment, the host computer for providing communication service to all other nodes more frequently generates a transmission request. If the token is assigned from a given host computer to another, the given host computer cannot obtain the transmission right again until the token is sent through all other nodes. Therefore, the host computer is kept in transmission wait status and the number of transmission requests continues to increase.
Even if the token is obtained again, it must be assigned to the next node before all transmission requests are handled, because of the limits of the maximum token occupied time.
According to the conventional token passing control method, the opportunity for communication service to a node with a large number of transmission requests is the same as that for communication service to other nodes. Therefore, this method cannot satisfy all transmission requests. In the worst case, the system operation is interrupted.